This invention relates to a conductive seal used for sealing an opening in a housing, especially for filling a gap between the housing of electronic equipment and its cover.
In related arts, various kinds of resins are used for a sealant. The sealant fits the gap between the housing and the cover using its rubber-like elasticity to secure the contents of the housing and prevent the contents from being interfered with from the outside. For example, an O ring made of synthetic resin or a synthetic resin packing adaptable to the configuration of the gap is used. Another known sealant is composed of pasted compound having adhesion and can also be used as an adhesive. In addition, a sealant for sealing joints in the housing of an automobile transmission, a sealant for shielding an automobile windowpane, and a sealant for sealing joints in an architectural panel are known.
The related-art seals and sealants having a rubber-like elasticity do not have conductivity. Consequently, when the seal or sealant is applied to the housing of the electronic equipment, electromagnetic waves leak in or out of the housing. Electromagnetic noise generated by the electronic equipment in the housing is transmitted outside, thus adversely affecting the equipment outside the housing. To solve this problem, the method for providing conductivity to a seal or sealant has been under consideration.
To provide conductivity to a sealant, metallic powder or carbon black can be mixed in the sealant. However, mixed solid particles like carbon black or metallic powder change the properties of sealants mainly composed of rubber or synthetic resin. If the amount of the solid particles mixed into the sealant is too small, the sealant has elongation, but it does not have tensile strength or shear strength. By mixing more solid particles into the sealant, however, the sealant becomes harder but more brittle. As a result, the sealant becomes fragile and has low flexural strength.
When a sealant is manufactured primarily for providing conductivity, a large amount of carbon black must be mixed into the sealant, thereby hardening the sealant too much. A sealant with such a composition cannot fit a gap or produce a sealing effect.
When a sufficient amount of carbon black for providing conductivity is mixed in unhardened sealant paste, the sealant becomes so viscous due to mixed carbon black that it cannot be applied to the gap to seal. To decrease the viscosity of the sealant, a solvent can be added. Even if the sealant can be applied to the gap by adding a solvent, however, the sealant is still too stiff to produce suitable sealing effect.
When metallic powder is used instead of carbon black, the sealant requires a large amount of metallic powder to enhance its conductivity, resulting in the same problem as carbon black. Moreover, since metallic powder has a high specific gravity, the specific gravity of the sealant is also increased. The required thixotropy of the sealant is thus deteriorated. When, to produce a sealing effect, the sealant is formed in a thick strand before it is applied to the gap, the sealant sags at the gap before it hardens, deteriorating the sealing effect.
Silver or copper is conductive, but expensive. It is economically unfeasible to mix a large amount of silver or copper in the sealant formed in thick strands.